South Dakota was won by Republican nominee John McCain by an 8.4% margin of victory. Prior to the election, all 17 news organizations considered this a state McCain would win, or otherwise considered as a red state. Like the other states located in the Great Plains region, South Dakota is a predominantly rural and sparsely populated state with conservative voting tendencies which favors the Republicans, who dominate elections at the state and federal level in the state. South Dakota stayed in the GOP column in 2008 as Republican John McCain carried the state with 53.16%.

South Dakota, a predominantly Republican state, has not voted for a Democratic presidential nominee since Lyndon B. Johnson won the state in the landslide Election of 1964. A sparsely populated state with a rural and conservative lifestyle of many of the state's inhabitants, since then, the state has been won handily by the Republicans.

McCain was able to keep South Dakota in the GOP column in 2008, taking in 53.16% of the total statewide vote over Obama who received 44.75%, an 8.41-percent margin of victory. This margin of victory was considerably smaller compared to 2004 when George W. Bush carried South Dakota with 59.91% of the vote over John Kerry who received 38.44%, a 21.47% margin of victory, resulting in a 13.06-percent swing to the Democrats in 2008.

While McCain did well throughout the state, his main strength was in Western South Dakota, where he often won by landslide margins.[21] He was able to carry Pennington County, South Dakota, which contains the state's second largest city of Rapid City. In contrast, Obama ran best in Eastern South Dakota, losing most counties by fairly close margins. He also did well among Native Americans; in Western South Dakota, the only counties Obama won were majority Native American.

Obama was able to substantially improve upon John Kerry's showing in South Dakota in 2004 by a number of factors. First, it helped that South Dakota received media attention during the course of the 2008 Democratic Primary, being the last state to vote in the historic and contentious primary that gave Hillary Rodham Clinton an 11-point victory over Obama; it was Clinton's last victory in the primary. In the general election, Obama was able to cut the margin significantly by narrowly carrying Minnehaha County, which contains the state's largest city of Sioux Falls. He was also able to win Brown County, which contains Aberdeen, as well as Brookings County which contains Brookings, home of South Dakota State University. He did much better than Kerry in Eastern South Dakota, which is where most of the people live, but McCain's margins throughout the state were too large to overcome.

South Dakota has only one congressional district because of its low population compared to other states. This district, called the At-Large district because it covers the entire state, is equivalent to the statewide election results.

Technically the voters of South Dakota cast their ballots for electors, representatives to the Electoral College. The state is allocated 3 electors because it has 1 congressional district and 2 senators. All candidates who appear on the ballot or qualify to receive write-in votes must submit a list of 3 electors, who pledge to vote for their candidate and the candidate's running mate. In the state's First Past the Post (plurality voting) system, the winner of the majority of votes in the state is awarded all 3 electoral votes. Their chosen electors then vote for President and Vice President. Although electors are pledged to their candidate and running mate, they are not obligated to vote for them.[22] An elector who votes for someone other than his or her candidate is known as a faithless elector.

The electors of each state and the District of Columbia met on December 15, 2008, to cast their votes for President and Vice President. The Electoral College itself never meets as one body. Instead the electors from each state and the District of Columbia met in their respective capitals.